Long lived power sources are needed for equipment that is too remote or inaccessible for replacement. By choosing a radioactive element with a long half life, we can create a long lived power source. The appropriate element should:

1. produce weakly penetrating radiation that can be easily shielded
2. a specific power of at least 0.2 kW/kg
3. have good corrosion resistance
4. be insoluble in water
5. be made of reasonably available material.

Among the transuranium elements, oxides of the alpha-emitting nuclides ²³⁸Pu (t1/ 2 = 87.7 years) and ²⁴⁴Cm (t_{1/ 2} = 18.1 years) are useful fuels. A few grams to kilograms of such nuclides, in appropriately shielded containers, provide intense sources of heat with power levels up to hundreds of watts, since the alpha particles are stopped very easily and their decay energy converted into thermal energy. Using thermoelectric devices without moving parts, it is possible to convert the resultant heat flow into usable electricity.

Such power sources are small, lightweight, and rugged. One of their uses is in the SNAP (Space Nuclear Auxiliary Power) units used to power satellites, or more importantly, to power remote sensing instrument packages. A satellite that used a SNAP source is shown in Fig. 13-5. SNAP sources (fueled by ²³⁸Pu) served as the power sources for instrument packages on the five Apollo missions, the Viking unmanned Mars lander, and the Pioneer, Voyager and Cassini probes to Jupiter, Saturn, Uranus, Neptune, Pluto, and beyond.

In addition to space applications, radionuclide power sources have been used as terrestrial sources of energy wherever compact and long-lived sources, not requiring maintenance, are needed. In the early 1970s, ²³⁸Pu batteries were used as the power sources for cardiac pacemakers. Over 3,500 units were implanted and most remain functioning. Due to a lower cost and easier construction, lithium batteries have largely supplanted the plutonium.